1. Field of the Invention
This invention relates to eye fundus cameras capable of selectively performing fluorescein angiography of the fundus of an eye and the color or monochromatic photography (called normal photography).
2. Description of the Prior Art
The angular fields of the non-contact type eye fundus cameras have for a long time conventionally been in the order of 30 degrees. Recently, however, as the design of wide angle objective lenses has advanced and the method of removing the disturbing reflections and scattering the illuminating light bundle has advanced, the angular field has exceeded 45 degrees. Since the use of the wide angle objective lens makes it possible to take photographs of a large area of the eye fundus in a reduced number of shots with a reduction in the required number of photographs for every specimen, it becomes easier to make photographs of almost equal image quality. The patient will have less pain and there is a great demand for increasing further the wide angular field.
One of the important factors for increasing the field angle is to eliminate the disturbing light. Since the photographic system and the illuminating system are coaxially arranged in the eye fundus camera, it has been a long established practice in the art that, in order to prevent a portion of the illuminating light reflected on the cornea to migrate into the photographic system, a control obscuring diaphragm is provided in the illuminating light path, thus shutting out the light which would otherwise impinge on the cornea. Since the objective lens has an angular field of 45.degree. or more, however, the presence of undesirable reflections on the crystalline lens poses a serious problem. In the conventional narrow angle eye fundus camera (30.degree.), the image of the shade of the obscuring diaphragm overlaps not only the reflective area of the cornea but also the crystalline lens. But the increase in the angular field of the objective lens calls for a decrease in the size of the shade of the obscuring diaphragm so that even the crystalline lens forms a source of undesirable light.
As the counter-measure for this, Japanese Patent Sho No. 51-24249 (U.S. Pat. No. 3,851,954) discloses that at the front surface, or at an intermediate point between the front and rear surfaces of the crystalline lens, there is formed a baffle image to eliminate the reflections on the crystalline lens. Japanese Laid Open Utility Model Sho No. 52-107140 (U.S. Pat. No. 4,102,563) discloses that the illuminating system is provided with a black point conjugate to the rear surface of the crystalline lens eliminating the scattering light within the crystalline lens and from the rear surface thereof.
Aside from this, in eye fundus cameras, a limitation is that the fundus of an eye to be examined is illuminated by a light bundle passing through the pupil thereof. The higher the level of illumination, the more desirable it is. It is, therefore, preferable to increase the diameter of the pupil. As the age of the patient increases, however, a sufficient dilation of the eye's pupil becomes more difficult even if a mydriatic is applied to the eye. Therefore, the selectable pupil diameter in actual practice must be considerably smaller than the maximum possible one. In the undesirable light eliminating method, as the angular field widens, the area which must be shielded from light increases, thereby giving rise to the problem that within the framework of the limited pupil diameter, total elimination of undesirable light becomes difficult. Accordingly, it is much desired to compromise the requirements of preserving the high image quality and of increasing the angular field, or decreasing the pupil diameter.
On the other hand, the ordinary eye fundus cameras, besides being capable of normal photography, are adaptable to fluorescein eye fundus angiography. Fluorescein angiography of the eye fundus is such that when the fluorescein sodium injected from an elbowvein reaches the blood circulation system in the eye, the ocular fundus is illuminated by an excited light of wavelengths of a nearly blue color so that as the blood columns are caused to give off fluorescein light, suitable photographic film is exposed to remove only the fluorescein light from the light rays returning from the eye to be examined. For this purpose, use is made of a barrier filter in the photographic ray path of light, and of an exciter in the illuminating ray path of light to separate the wavelengths.
The present inventors have, in the preceding Japanese Laid Open Patent Sho No. 55-26959, considered that when in fluorescein angiography the illuminating light and the photographing light are differentiated in wavelength from each other, a proposal is made that the baffle for removing the disturbing light has to be demounted with the advantage that the illumination level is increased. Since, as a result of the wavelength separation, the rays of light which have wavelengths in the illuminating wavelength region are blocked by the barrier filter, it doesn't matter if a disturbing light is produced provided that it has a wavelength in the region, since this does not lead to badly deteriorate the image quality. Another arrangement for eliminating the function of the baffle is also disclosed in U.S. patent application Ser. No. 32,683.